A battery can deliver a current to a load such as an engine, a light or a radio for a considerable length of time. As long as the potential difference between its terminals remains close to a nominal value and a sufficient current is generated, current will flow to the load. As the current flows, the battery's capacity to drive the load becomes diminished and may eventually become exhausted.
To replenish this loss, a battery may be connected to a source that provides positive and negative charges to its electrodes. The charges are transported from one terminal to another in a direction that is opposite to the bias of the terminals. A device that accomplishes this transport may be referred to as a battery charger. Battery chargers provide a current to and a voltage across the battery in accordance with an algorithm or “charge profile.”
For lead-acid batteries, charging generally takes place in three separate steps or stages: bulk, absorption, and float. The first step in the battery charging process is the bulk stage, or constant current charging. During this stage, the input current to the battery is set at the maximum safe rate the battery will accept, or the maximum that the charger can supply, until voltage rises to near the fully charged level (e.g., 75-80% of final charge). Once the battery is charged to 75-80% of its final charge, the battery charger may be moved to the absorption charging stage, also sometimes referred to as the constant voltage charging stage. During this stage, the input voltage to the battery is held constant while the current is gradually tapered off. Omitting this step (i.e., simply using the battery at the 75-80% state of charge) can eventually cause some batteries to lose their ability to accept a full charge. The third and final step in the three stage battery charging process is the float phase, or sometimes referred to as the maintenance phase. After a battery reaches its full charge during the absorption stage, the input voltage is lowered to a reduced level. This step compensates for the battery's tendency to self-discharge, and further allows for the reduction in gassing, as well as helping to prolong the battery's life. In general, for lead-acid batteries oxygen and hydrogen gas will be released at recharge voltages of between 13.8 volts and 14.2 volts.
On a 12 volt battery, the no load battery voltage is between 11.4 volts, fully discharged, and 12.9 to 13.0 volts, fully charged. When the battery charger is connected to the battery and then turned on by plugging it in to a power source, the charger will operate in the bulk stage and will attempt to bring the battery voltage up to the level required to be in absorption stage. Generally, this voltage may be in the 14.2 to 15.0 volt range. The actual voltages may depend on various factors including type of battery (e.g., gel, absorbed glass-mat (AGM)), age of battery, etc.).
The transition from absorption stage to the float stage is determined either by a timer, or by the battery charger sensing the value of charge current and then switching over when the charge current drops below a certain threshold. For example, a battery charger may switch out of absorption mode when the charge current falls below 5 amps, or when the absorption mode has lasted for at least eight hours.
In the float stage, the magnitude of the voltage may be a few tenths of a volt above the no load, fully charged voltage, e.g., between 13.2 and 13.6 volts.
In some applications, it is desirable to have a battery charger with multiple outputs to facilitate charging multiple batteries (or multiple battery banks) simultaneously. One type of such battery charger is a switched-mode multi-output battery charger. These types of battery chargers may be constructed as multiple switch-mode converters, single converters with isolated secondary windings, or single converters with output distribution circuitry. Single converter chargers with output distribution circuitry are often chosen for higher capacity charging applications (e.g., 25 amps and higher) because the converter circuitry can be used most efficiently to supply the needs of a primary battery while also meeting the needs of one or more auxiliary batteries.